Electron paramagnetic resonance studies on a single crystal of Cu2+-doped monopyrazine zinc sulphate trihydrate (PZST) have been made over an extended temperature range (4.2-375 K). Three physically equivalent, but magnetically inequivalent, Cu2+ complexes have been observed. The spin-Hamiltonian parameters are rigorously evaluated at 334, 295, 77 and 4.2 K, by the method of least-squares fitting, utilising numerical diagonalisation of the spin-Hamiltonian matrix on a digital computer. The principal values of the g- and A-tensors indicate that, in PZST, the Cu2+ ion experiences an octahedral symmetry with orthorhombic distortion. The data are interpreted to conclude the occurrence of both static and dynamic Jahn-Teller effects over the temperature range of investigation, the transition from static to dynamic Jahn-Teller effect occurring at 334+or-1 K.

The spin-ordering and electronic properties of the c(2 × 2) MnCu/Cu(1 1 0) surface alloy are investigated by means of ab initio density-functional calculations. We first address the magnetic ground state and the robustness of the spin-polarization properties. The lowest-energy state is found to be ferromagnetic with a very low Curie temperature, showing that the paramagnetic phase should be established in this system at room temperature. The calculated trends obtained for the various spin structures considered indicate that the local Mn-spin moment and resulting reduced work function as well as Mn-outward buckling should persist in the paramagnetic phase. We then address the electronic surface-band structure of the paramagnetic phase close to the Fermi energy, in connection with the interpretation of recent angle-resolved-photoemission-spectroscopy experiments at room temperature. Our calculations account for an intriguing new surface-band feature observed experimentally near the [Formula: see text] point upon alloy formation, and provide a microscopic interpretation for this feature and for the alloy-induced changes in the Cu(1 1 0) Shockley surface state.